P. falciparum, an intraerythrocytic parasite, obtains nourishment primarily through phagocytosis of the host cytosol but also through the incorporation of extracellular small molecules which enter through the parasitized red cell's membrane via pores. Normal mature erythrocytes are incapable of endocytosis. Several lines of evidence suggest that extracellular large molecules may be taken up when the mature red cell is parasitized by P. falciparum, but direct evidence has been lacking. We now report the use of ferritin, an electron dense protein, to demonstrate endocytosis in P. falciparum infected red cells. Parasitized red cells incubated with ferritin internalize that macromolecule as demonstrated by electron microscopy. While normal red cells incubated with ferritin took up none of the tracer molecule, parasitized red cells internalized substantial amounts. In addition both ferritin and apoferritin inhibited the growth of P. falciparum in a dose dependent fashion, again indicating endocytosis of a macromolecule. These data indicate that P. falciparum can somehow stimulate the mature erythrocyte to engage in endocytosis. We also note that both infected and non-infected red cells in a culture in which P. falciparum is growing become abnormally sticky for ferritin. Moreover, parasitized red cells bind I125-transferrin while non-parasitized erythrocytes do not. These observations suggest that a soluble parasite product alters the red cell membrane in a non-global manner, causing selective effects in relation to different proteins.